1. Field of the Invention
This invention relates generally to database management systems and, more particularly, to efficient evaluation of subqueries in SQL statements processed in relational database management systems.
2. Description of the Related Art
Information is frequently stored in computer processing systems in the form of a relational database. A relational database stores information as a collection of tables having interrelated columns and rows. A relational database management system (RDBMS) provides a user interface to store and retrieve the information and provides a query methodology that permits table operations to be performed on the data. One such RDBMS interface is the Structured Query Language (SQL) interface, which is specified by standards adopted by the American National Standards Institute (ANSI) and the International Standards Organization (ISO) following original development work by the International Business Machines (IBM) Corporation. The SQL interface permits users to formulate operations on the data tables either interactively, or through batch file processing, or embedded in host languages such as C, COBOL, or the like.
In particular, SQL provides table operations with which users can request database information and form one or more new tables out of the operation results. Data from multiple tables, or views, can be linked to perform complex sets of table operations with a single statement. The table operations are specified in SQL statements called queries. One typical SQL operation in a query is the "SELECT" operation, which retrieves table rows and columns that meet a specified selection parameter. Another operation permitted by SQL is the "JOIN" operation, which concatenates all or part of two or more tables to create a new resulting table. For example, a query might produce a table that contains the names of all supervisory employees who live in a given city, and might do so by specifying a SELECT operation to retrieve employee names and resident cities from one table, and then performing a JOIN of that data after a SELECT operation to retrieve employee names and job titles from another table.